Quote Originally Posted by Donald Miller
I have a 138S as well. I haven't had to deal with the conversion yet. I still have a 300 watt and a 500 watt Thorn lamp. Eventually, obviously the time will come. What I would do is use a minimum of 1000 watt halogen lamp and use the same principal that Jensen Optical did with their $800 upgrade. I would build a reflector that would be placed to the rear of the halogen lamp. The material that I would use for the reflector is Mirro which has above 90% reflective qualities. The reflector would need to be curved and emulate the radius of the halogen lamp.
Durst built a thing called "LAVAKO". It was used to adapt the Agfa Varioscope color head (made in the '50s and '60s) to the L138. You had to remove the mirror, insert the LAVAKO (essentially just a tin box with sides cut 45deg) and mount the color head on top of the whole thing. The light from the mixing box in the color head went down in the top condensor, you could use all neg sizes when using a 6x9 mixing box (using different condensors, of course). The knob at the front was used to adjust the distance from light source to condensor to optimize lighting.

I adapted a 100W Meopta color head to it and used it for some time, replacing it with a real Durst color head eventually.